The present invention relates to a control system for automatically turning on and off vehicular head lamps.
As this type of a vehicular head lamps control system, there has heretofore been known, for example, Japanese Utility Model Laid-open Publication No. 58-2139, a control system composed of a photosensor for detecting the quantity of light around a vehicle and producing a detected signal; a reference signal generating means for generating a reference light quantity signal corresponding to the darkness required for lighting vehicular head lamps; an output signal generating means adapted to operate upon receipt of a supply voltage from a DC power source of the vehicle, compare the value of the detected signal with that of the reference signal and produce an output signal when it judges that the outside of the vehicle is dark; a flip flop circuit responsive to closing of an ignition switch of the vehicle to receive at an input terminal thereof the supply voltage from the DC power source through the ignition switch and a first fuse and produce a supply voltage retaining signal from an output terminal thereof; an auxiliary power supply means adapted to operate upon receipt at a control terminal thereof the supply voltage retaining signal from the flip-flop circuit, receive at an input terminal thereof the supply voltage from the DC power source through a second fuse and provide this supply voltage from an output terminal thereof to the output signal generating means; and drive means for lighting the head lamps in response to said output signal and turning off the head lamps in response to the extinction of said output signal. In such a head lamps control system, usually a plurality of different loads are connected to power supply lines via the first and second fuses.
In the thus-constructed vehicular head lamps control system, when the second fuse is in a normal condition, the head lamps can be automatically turned on and off according to the brightness of the exterior of the vehicle by the supply of power from the vehicular battery via the second fuse. However, in the event of a sudden failure of the second fuse due to some trouble of a load via the second fuse or bite-in of the power supply line extending from the second fuse to the flip flop circuit, there arises the problem that, although this system is in a normal condition, the supply of power is cut off, thus making impossible the normal automatic lighting and turning off of the head lamps. This means that in the event the second fuse suddenly opens while the vehicle is running under the light of the head lamps during the night, the head lamps suddenly come off against the driver's will. As a result, a smooth running of the vehicle is impeded.